1. Field of the Invention
The present invention relates to a liquid crystal element having a composite film or layer formed of liquid crystal and resin.
2. Description of the Related Art
In recent years, a liquid crystal element having a composite material formed of liquid crystal and resin has been proposed as a kind of liquid crystal element used for displaying characters and images. For example, U.S. Pat. No. 5,437,811 has taught a liquid crystal element in which a composite layer is retained between a pair of plates. The composite film or layer is formed of chiral nematic liquid crystal exhibiting a cholesteric phase and dispersed in a resin matrix.
The liquid crystal element, in which the liquid crystal exhibiting the cholesteric phase is dispersed in the resin matrix, utilizes selective reflection of incident light by the liquid crystal, so that it does not require a polarizer, and therefore can perform bright reflective display.
Since the liquid crystal element, in which the liquid crystal exhibiting the cholesteric phase is dispersed in the resin matrix, has a memory effect, it does not require a memory element such as TFT, MIM or the like, and therefore can perform display at high resolution.
Under a no field condition, the liquid crystal material of this element selectively attains a planar orientation or state, in which helical axes of domains consisting of liquid crystal molecules are aligned perpendicularly to the substrates, or a focal-conic orientation or state, in which helical axes of domains consisting of liquid crystal molecules are directed irregularly. When a relatively high voltage in a pulse form is applied to the composite layer in the focal-conic state, the liquid crystal molecules align to a direction of the voltage application and attain the homeotropic state. Then, by ceasing the voltage application, the liquid crystal molecules form domains again, and attain the planar state. Alternatively, when a relatively low voltage in a pulse form is applied to the composite layer in the planar state, the liquid crystal molecules align to a direction of the voltage application and attain the homeotropic state. Then, by ceasing the voltage application, the liquid crystal molecules form domains again, and attain the focal-conic state.
It is considered that the bistability of these two states is achieved by the fact that the resin restricts movement of the liquid crystal.
The liquid crystal exhibiting the cholesteric phase selectively reflects light beams of a wavelength corresponding to a product of a helical pitch length and an average refractive index of the liquid crystal, when it is in the planar orientation. By setting the selective reflection wavelength in a visible range, therefore, the liquid crystal in the planar orientation can display an arbitrary color. When the above setting is employed, the liquid crystal in the focal-conic orientation exhibits a transparent appearance. Therefore, display in a colored state and a transparent state can be selectively achieved in accordance with the selective reflection wavelength. For example, display in a colored state corresponding to the selective reflection wavelength and a black state can be performed by utilizing a black background color.
When the selective reflection wavelength of the liquid crystal is set in the invisible range such as an infrared range, the liquid crystal in the planar orientation transmits the visible rays and therefore exhibits a transparent appearance. In the focal-conic orientation, it scatters the incident rays and therefore exhibits a white opaque appearance. Therefore, monochrome display can be performed by utilizing a black background color. Also, display in an arbitrary colored state and a white state can be performed by utilizing the arbitrary background color.
However, if the selective reflection wavelength is set in the invisible range such as an infrared range, such a problem arises that a transmittance or transmission factor of visual rays in the planar orientation decreases with increase in helical pitch length, resulting in reduction in contrast.
The liquid crystal having the composite layer, in which liquid crystal exhibiting a cholesteric phase is dispersed in the resin matrix, has transmittance in the planar and focal-conic orientations as well as stability in these orientations which depend on the kind of the liquid crystal (kinds and a mixture rate of liquid crystal materials in the case where the liquid crystal exhibiting a cholesteric phase is formed of a mixture of two or more kinds of liquid crystal materials) as well as kinds of the resin or the like. However, the liquid crystal element exhibiting a sufficient contrast between the planar orientation and focal-conic orientation has not yet been developed.